I just got back from a superb Wineskills session at Denbies where Dr Mike Trought (English bloke who works in academia in Marlborough, New Zealand) gave a day’s session on “grapevine yield mangement and its influence on fruit composition.”

I can’t speak for Mike in my notes here and I may well be misquoting him, but here are some interesting points from the notes I took. With that in mind if anyone sees inaccuracies then please shout and I’ll make amends!

There was a time in NZ of grape over-production in the mid 1980s, with the planted area falling off towards 1990. However, what followed this was the current explosive period or planting which was many times bigger than before. This lead to the current problem of over-production, particularly of Marlborough Sauvignon Blanc. This variety makes up 60% of NZ output, and at 31,000 acres is almost half the area dedicated to the same variety in France. Prices of Sauvignon Blanc in NZ halved in 2 years from NZ$2500 to NZ$1250 a tonne (actually I’m sure a friend of mine who worked there told me that prices had fallen to a quarter of what they were). Some growers are now having to limit production to 10-12 t/ha to manage supply with wine producers who just cannot take the grapes. There is little official legislation/regulation in NZ, with growers and wine makers largely free to choose what and how to produce. The typical planting density for Marlborough growers is around 2,200 vines/acre (spacing 2.5m x 1.8m).

Gisbourne is the most similar region in NZ to England when considering the climate. Mike drew climate comparisons between Marlborough and Kent in terms of rainfall and temperature (both marginally better in NZ) but showed that the sunshine hours were dramatically higher in Marlborough. He also emphasised how important the year’s distribution of weather was (e.g. in England it tend to rain little and often (hence disease) compared to NZ wine growing regions), and that the temperature during current season’s flowering period is a very important determinant of the yield, probably since pollen tube growth is highly dependent on temperature (Mike also dropped in the fact that most fertilisation happens before cap-fall). Specifically we were shown a model with a high correlation indicating that average bunch weight is a function of the temperature at flowering. Mike also stressed the importance of the weather in one year (at time of floral initiation) in the next year’s harvest, especially in determining the number of bunches per shoot. However, although floral initiation starts at around flowering time it goes on long after that with each subsequent node undergoing floral initiation at roughly 4 week intervals.

Mike went though some very interesting work he has done relating historic yields and weather. This is of particular interest to me since I have already undertaken the first draft of an analogous study on 10 years of English (Plumpton College) yield data. I am currently working with several commercial growers to integrate their data). I’m encouraged (and also a little surprised) to say that a lot of the concepts I applied in my initial work are very similar to Mike’s! I caught up with him afterwards to hopefully initiate an ongoing dialogue where I can apply his best-practice to English data. In his climate-v-yield modelling Mike uses the concept of cumulative growing-degree-days (GDD), and looks at the deviation of the current season’s metric compared to long term average. Unsurprisingly he finds that in years with a higher cumulative GDD the yield is higher. Specifically, Mike’s model (not yet published, sadly) gives yield predictions as an addition of scaled average temperatures at time of floral initiation plus scaled average temperatures during flowering (minus a constant). He even told us the scaling factors and the constant but it’s not my place to mention those here (note – after having tried to examine this further I’m not sure it makes sense – I think I probably copied it down wrongly – damn). That all sounds interesting, but I am intrigued by the lack of parameterisation for subsequent growing season temperatures or for rainfall for example. Interestingly, throughout the day Mike referred to the importance of cane diameter in the results of his studies, and in fact he mentioned that he thought adding can diameter as a parameter to his model was likely to improve its predictive powers. He certainly said that bunch number increases with cane diameter.

The typical time from flowering to harvest is 125 days. However a delayed flowering date usually results in a slight compression of this period (in my language the harvest date is slightly ‘sticky’).

Throughout Mike stressed the importance of good record keeping in vineyard management.

Mike suggested 4 ways in which yield manifests itself: shoots/hectare, bunches/shoot, berries/bunch and bunch weight. What was nice here was that much of the information Mike presented matched what I’ve learned to date during my time at Plumpton (well done Chris Foss). I won’t reproduce it here! I don’t pretend to understand it all but at least it’s somewhat familiar. I’ll report on a few things of interest I jotted down, though. Mike thinks vine turgidity just after flowering is a major determinant of berry weight. Also we heard the view that grape sugar levels at harvest are a good indicator of a vine’s reserves, and that more reserves at bud-break would lead to more bunches.

Mike’s not too much a fan of arching canes in an attempt to induce even growth of shoots along a cane (re end-point principal of shoot growth). This point of view is from the practical management perspective of the lack of subsequent uniformity of shoot heights which makes canopy management by lifting fruiting wires tricky. He’d rather see growers control vigour in other ways such as cover crops or install additional fruiting wires and laying down more canes. Quadruple guyot, anyone?

Regarding cane v spur pruning the view was expressed that cane pruning is best for England’s marginal climate, mainly due to improved fruitfulness of this method. Although he accepted that spur pruning leaves more old wood and hence more reserves, he said this was a marginal consideration since most vine reserves are in the roots and not the old wood. These reserves, in a cool climate, are really laid down before harvest (no real post-harvest recovery period), in contrast to warmer growing regions such as California.

Laterals should be removed, Mike says, and care taken not to generate too many when trimming shoots. Shoots with less than 8 leaves are a net drain on the vine. Removing younger leaves stimulates older leaves to photosynthesise harder (that’s a new concept to me). A green harvest of 20% was also recommended, with workers going through the vineyard at veraision and removing the 20% most unripe fruit (this also helps for the homogeneity of ripening at harvest). Another interesting soundbite: removing a leaf on a node will reduce the fruitfulness of shoots from that node in the following year. And another factoid: regarding frost protection, Marlborough viticulturalists reckon on a +0.2C change in every +10cm height in fruiting wire.

Well, there was a lot more to take in but my head had kind of exploded by mid-afternoon. The session closed with me feeling I had more to learn than when I had started, but that’s a good thing, right? A great session, Mike – thanks very much for doing it.